The present invention relates to tooling using thermostructural composite materials containing carbon for installations for performing heat treatment and/or thermochemical treatment. The term "tooling" is used herein to cover those elements which serve to support other elements that are to be treated in such installations, in particular oven sole-plates, loading trays, legs and spacers for holding trays at different levels, etc.
Thermostructural composite materials are characterized by their mechanical properties that make them suitable for constituting structural elements, and by their ability to retain these properties at high temperatures. They have various uses, in particular as fairing elements for space vehicles, friction disks, in particular aircraft brake disks, parts for thruster nozzles, parts for turbojets, or indeed tooling for ovens in which treatments are performed at high temperature. Examples of thermostructural composite materials are materials comprising a preform, i.e. fiber reinforcement made of refractory fibers (carbon or ceramic fibers) densified by a matrix that is likewise refractory (carbon or ceramic), such as carbon--carbon composite materials (C--C, for carbon fiber reinforcement and a carbon matrix), or ceramic matrix composite materials (CMCs).
The use of thermostructural composite materials, and more particularly of C--C composite materials for tooling that is intended for heat treatment installations is described, in particular, in document FR-A-2 677 740.
Compared with metal tooling, such C--C composite tooling has the advantages of characteristics that do not deteriorate at high temperatures, and in particular, while at such temperatures, they are not subject to brittleness, creep, or deformation, and in addition they are of substantially lower mass, thereby providing an excellent ratio between the weight of the parts treated and the weight of the tooling.
Nevertheless, in addition to their relatively high cost, C--C composite materials, and more generally thermostructural composite materials containing carbon, can give rise to problems in certain applications.
They have a surface state characterized by a degree of roughness and conserved residual pores however much care is taken when densifying the fiber reinforcement with the matrix. Also, C--C composites are sensitive to corrosion by oxidation in air at temperatures above 450.degree. C. This sensitivity to oxidation also exists for CMCs, either because the fiber reinforcement is made of carbon fibers, or else because an interphase layer of pyrolytic carbon is formed between the fibers of the reinforcement and the matrix, in the manner described in particular in document EP-A-0 172 082. Also, when the tooling is used in installations for treating metal elements, the presence of carbon in the thermostructural materials can give rise to undesirable phenomena of the treated elements being carbided.
It is therefore desirable to remedy these drawbacks so as to be able to benefit fully from the advantages offered by thermostructural composite materials, and the solution proposed herein consists in making the tooling in the form of composite material parts provided with metal covering.
The composite part and the metal case could be manufactured separately and then assembled together. However, that technique is not envisagable in practice because of the complexity of assembling the parts and because of the difficulty in controlling the large clearance that inevitably appears at high temperatures between the composite part and its case. Another possibility lies in hot-forming the metal case, e.g. by casting metal around the composite part. In addition to the problem caused by the difference between the coefficients of thermal expansion of the composite part and of the metal case with regard to the necessity to conserve the integrity thereof, contact between metal and composite material containing carbon also suffers from drawbacks. In particular when the metal case is made of steel, eutetics may form with the carbon of the composite part, having the undesirable consequence of degrading the mechanical behavior of the tooling.
Document FR-A-2 676 051 has indeed proposed making a metal covering from a mixture of powders deposited on the surface of a part and subjected to heat treatment, however there still remains the main drawback mentioned above of methods in which the metal case is formed hot.